2.2 Filters

Light filters may also be placed into the optical path. Simple filters placed between the light source and the condenser change the colour or intensity of the light, which is often useful when photographing sections. The human eye is much more accommodating of variations in light intensity and quality than a digital camera.

Very occasionally polarised light is used to examine histological specimens. It requires a polarising filter below the condenser and a second polarising filter above the section. For example, individuals with gout have crystals of uric acid within the joint, which are difficult to distinguish by bright field microscopy, but the crystals themselves polarise light. If the two polarising filters are set at 90° to each other so no light should come through the microscope, the uric acid crystals show up bright on a dark background (Figure 5).

Figure 5 Crystals of monosodium urate are present in the joint fluid of patients with gout, and are diagnostic. The crystal contained within an inflammatory cell is not easily distinguished under normal illumination (left) but the crystal is highly birefringent, and is easily seen using illumination with crossed polarisation filters (right).